Can I use Project from view while having several images for different sides?
What Ian Hubert does is a bit punky, but as his characters are far away it works fine. Basically I think he projects his mesh from view, then moves the vertices in the UV Editor so that the UV matches as much as possible with the picture:

Then, for the sides and back, he probably unwrap this faces and put the UV islands over a part of the picture that will texture correctly this part of the body:

At the end, if you plan to paint in Weight Paint mode to bring some corrections to the projected texture, duplicate your object, unwrap again, but this time with the Smart UV Project option (with a enough margin), give it a black image texture, bake the original. Now you can paint over the new object, for example with the Clone brush, without affecting the original picture, which could cause problems because overlapping UVs: